Tomography is a rapidly advancing imaging technology with broad applications in such varied fields such as, for example, medicine, dentistry, biology, environmental, toxicology, mineralogy, and electronics. Tomography is the process of forming a 3-D image of a sample using various tools, such as, an x-ray system, a transmission electron microscope (TEM), scanning transmission electron microscope (STEM), and/or atom probe microscopy (APM) to obtain various types of information such as, for example, atomic structure and chemical analysis of the sample. A 3-D tomography dataset is typically obtained from a photon (e.g., optical or x-ray) or electron microscope by reconstructing a series of 2-D projection images taken through the sample at different angles, or in the case of an APM by reconstructing a volume from a sequence of field-evaporated atoms striking a position-sensitive detector.
TEMs and STEMS allow observers to see extremely small features, on the order of nanometers, and allow analysis of the internal structure of a sample. For convenience, the reference to TEMs and STEMs will be indicated by the term “S/TEM” and references to preparing a sample for an S/TEM are to be understood to include preparing a sample for viewing on a TEM or a STEM. The sample must be sufficiently thin to allow many of the electrons in the beam to travel though the sample and exit on the opposite side. Thin S/TEM samples are typically cut from a bulk sample material and are known as “lamellae”. Lamellae are typically less than 100 nm thick, but for some applications a lamella must be considerably thinner. In S/TEM tomography, an electron beam is passed through the lamella at incremental degrees of rotation to form a series of tilted two-dimensional projections through a thin sample from which a three-dimensional rendering of the original structure can be constructed.
Atomic probe microscopes (APMs) typically include a sample mount, an electrode, and a detector. During analysis, a sample is carried by the specimen mount and a positive electrical charge is applied to the sample. The sample is typically in the form of a pillar having a narrowed needle-shaped tip. The detector is spaced from the sample and is either grounded or negatively charged. The electrode is located between the sample and the detector, and is either grounded or negatively charged. An electrical pulse and/or laser pulse is intermittently applied to the sample to cause atoms at the tip of the needle to ionize and separate or “evaporate” from the sample. The ionized atoms, molecules, or clusters-of-atoms pass through an aperture in the electrode and impact the surface of the detector resulting in a detected ion or a “count.” The elemental identity of an ionized atom can be determined by measuring its time of flight between the needle surface and the detector, which varies based on the mass/charge ratio of the ionized atom. The location of the ionized atom on the surface of the needle can be determined by measuring the location of the atom's impact on the detector. Accordingly, as the sample is evaporated, a three-dimensional map of the sample's constituents can be constructed.
S/TEM provides better structural data while APM provides better compositional data. The different tomographic data from either S/TEM or APM used alone prevents optimal material analysis. Correlative S/TEM and APM tomography utilizes data from both S/TEM and APM to obtain valuable structural and chemical information from the sample. The quality of data can vary depending on various aspects of the sample, such as, for example, size, shape, and density, and the composition and spatial distribution of features in the volume being analyzed. Current correlative S/TEM and APM tomography typically uses pillar-shaped needle samples that are nominally cylindrical and containing a region of interest (ROI). The quality of each individual S/TEM image in a tomographic series from a pillar sample is somewhat lower than can be achieved with a lamella sample as a result of, for example sample thickness and feature-obscuring projection effects. The quality of data from APM tomography data acquisition experiment depends largely on the three-dimensional arrangement of elements across the different areas of the sample. In general, an APM sample is elementally non-homogeneous, practically resulting in a sample having an indiscriminate number of and distribution of evaporation fields across the field-evaporating portion of the sample, each region of which must form a nominally hemispherical shape to simultaneously satisfy the basic equation governing field evaporation: Ei=kV/ri, where Ei is the evaporation field of the ith element of the surface of the APM sample, ri is the radius of the ith element of the surface of the APM sample, V is the voltage applied to the sample at any particular time in the data acquisition experiment, and k is a constant of proportionality that largely depends on the geometry of the electrode and sample. In the case where any one or more of the field-evaporating elements on the surface of the APM sample is unable to satisfy the requirements of the field-evaporation equation, the sample may spontaneously evaporate in an uncontrollable fashion (uncorrelated with voltage or laser pulse), leading to artifacts in the data or a catastrophic fracture of the sample. Additionally, it can be difficult to form a pillar sample with invisible or buried features of the ROI properly positioned within the pillar. Yet another drawback to correlating S/TEM and APM using a pillar sample is that with the field-of-view of an APM dataset is limited to approximately the inner 50% of the formed pillar shaped sample, a compromise exists between S/TEM data quality and analysis volume in the APM. Additionally, for those APM runs that make it through the ROI successfully, there are often significant distortions and artifacts in the raw data that cannot be adequately corrected for during the reconstruction or data rendering phase of the analysis. Current issues in APM data acquisition, reconstruction and analysis are described, for example, in Larson et al., “Atom probe tomography spatial reconstruction: Status and directions” Current Opinion in Solid State and Materials Science 17 (2013 pp. 236-247). Another problem with correlative S/TEM and APM tomography is that advanced S/TEM imaging and analytical techniques, such as, for example, holography, differential phase contrast, phase-plate contrast enhancement and even lattice imaging and analysis, are largely incompatible with a pillar-shaped sample.